Electronic devices use power to operate. Switched mode power supplies are commonly used due to their high efficiency, small size and low weight to power many of today's electronics. Conventional wall sockets provide a high voltage alternating current. In a switching power supply a high voltage alternating current (ac) input is converted to provide a well regulated direct current (dc) output through an energy transfer element. A typical switching power supply also comprises a switch coupled to the energy transfer element and a power supply control circuit coupled to the switch. The switched mode power supply control circuit usually regulates an output voltage of the power supply, output current of the power supply, or a combination of the two by sensing the output and controlling it in a closed loop. In operation, the switch is utilized to provide the desired output by varying the duty cycle (typically the ratio of the on-time of the switch to the total switching period) of the switch in a switched mode power supply.
A buck converter is one type of switching power supply where the duty cycle is substantially the ratio of the output voltage of the switching power supply to the input voltage when operating in continuous current mode. As such the ratio of the on-time and the off-time of the switch determines the output voltage. For loads which require a small output voltage in comparison to the input voltage, the duty cycle of the buck converter is small and as a result the on-time of the switch is small in comparison to the total switching period. For example, a power supply with an output voltage of 12 V from a rectified ac input voltage of 375 V would require an on-time which is 3.2% of the total switching period. For such cases, a tapped buck converter can provide the same output voltage to input voltage ratio as a buck converter but with a larger switch duty cycle. A larger switch duty cycle is desirable to reduce losses in the switch (typically a MOSFET, bipolar transistor or the like) that is coupled to the energy transfer element of the power supply
In a typical tapped buck converter configuration one end of an inductor is coupled to the switch, while the other end of the inductor is coupled to the output. A freewheeling diode is then coupled to a tap included in the inductor. A circuit may also be included in the switching power supply to provide a feedback signal that is representative of the output of the switching power supply. This feedback signal may then be used by the power supply control circuit to control the switching of the switch to regulate the output of the switching power supply. However, since the power supply output and power supply control circuit are referenced to different voltage levels in a tapped buck converter, the feedback signal needs to be level shifted in order to interface with the control circuit. Thus, the typical tapped buck converter configuration may include additional and relatively expensive circuitry for level shifting the feedback signal in this way. For example, a conventional tapped buck converter configuration may include an optocoupler or bias winding coupled between the output of the switching power supply and the power supply control circuit to level shift the feedback signal.